1. Field of the Invention
The present invention relates to a developing apparatus for making visible a latent image carried on a carrier, and more particularly, to a developing apparatus improved so that scattering of developer from the developing apparatus is prevented.
2. Description of the Related Art
An electrophotographic image forming apparatus such as a copier and a printer has a rotary drum having a photoreceptor as a carrier, and a developing apparatus is disposed opposite to the rotary drum. On the surface of the photoreceptor of the rotary drum, an electrostatic latent image is formed by electrophotography. The developing apparatus applies developer containing toner as a colorant to the electrostatic latent image formed on the photoreceptor, so that the toner adheres to the electrostatic latent image to make the latent image visible.
The toner image developed by th e d eve loping apparatus is transferred to a sheet of paper as a transfer material. After transfer, some toner that cannot be transferred remains on the photosensitive surface. The remaining unnecessary toner is removed from the surface of the photoreceptor before successively performing the next image formation. For that purpose, a cleaner is provided for removing the toner remaining on the surface of the photoreceptor. The unnecessary toner removed by the cleaner is collected into a collecting portion in the cleaner.
Such a developing apparatus has a development housing disposed opposite to the photoreceptor on which an electrostatic latent image is formed, and a developing roller which is rotated in a predetermined direction is disposed in the development housing. The developing roller holds developer in the development housing and conveys the developer to a development area that is opposite to the photoreceptor member. The developer carried by the developing roller acts on the surface of the photoreceptor in the development area. The development housing has an opening in correspondence with the development area. The developer carried by the developing roller is applied to the surface of the photoreceptor through the opening. After the electrostatic latent image is developed, the developer carried by the developing roller is collected into the development housing, mixed with the developer in the development housing, and used again.
However, there is a possibility that after development, some of the developer carried by the developing roller scatters outside through a gap between the development housing and the photoreceptor. When the developer scatters, the scattering developer adheres to a sheet of paper that is being conveyed to form an image thereon, or adheres to a guide member for guiding the conveyance of the sheet, so that the reverse surface of the sheet is smudged with the developer.
Examples of known developing apparatuses improved so that the scattering of the developer after development is prevented include developing apparatuses disclosed in Japanese Unexamined Patent Publication JP-A 4-107485 (1992) and Japanese Unexamined Utility Model Publication JP-U 4-70663 (1992).
In the developing apparatus described in the former prior art, a gap t between the bottom surface of the development housing and the developing roller is greater than a thickness t1 of a layer of the developer adhering to the developing roller. Moreover, by setting the gap t to be as close to t1 as possible, an air flow drawn into the development housing is caused by rotation of the developing roller, whereby developer which is about to scatter is collected into the development housing.
In the developing apparatus described in the latter prior art, a gap ratio t2/t of a gap t2 between the downstream side wall surface of the development housing and the developing roller to the gap t between the developing roller and the bottom surface of the development housing is not less than 0.7 and not more than 1.5. By setting the ratio like this, the scattering developer is drawn into the development housing by the rotation of the developing roller.
These prior art developing apparatuses produce their effects. However, when development is performed, the photoreceptor carrying an electrostatic latent image to be developed rotates as well as the developing roller of the developing apparatus. Consequently, an air flow is caused by the rotation of the photoreceptor, so that the developer which cannot be held by the developing roller scatters out of the development housing with the air flow.
In particular, the photoreceptor on the surface of the rotary drum is rotatively moved at a speed which depends on an image formation speed, and the rotative movement speed increases as the image formation speed increases. In an apparatus which performs image formation on, for example, 60 or more sheets per minute, when it is assumed that image formation can be performed on one sheet every rotation of the rotary drum, the rotative movement speed of the photoreceptor is 60 or more rotations per minute.
When the photoreceptor is moved at such a high speed, it is necessary that developer sufficient for contribution to development be supplied by the developing roller in the developing apparatus. For this reason, the rotation speed of the developing roller is increased as the image formation speed is increased. Consequently, the amount of the developer which cannot be held by the developing roller at the time of development increases, so that such developer scatters out of the developing apparatus with an air flow in the rotation direction of the photoreceptor. Such scattering of the developer cannot be prevented in the prior art developing apparatuses in any way.